Remarkable Structural Effect on the GoldâHydrogen Analogy in

Subscriber access provided by Macquarie University

A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Remarkable Structural Effect on the GoldHydrogen Analogy in Hydrogen-Doped Gold Cluster . Megha, Chinnathambi Kamal, Krishnakanta Mondal, Tapan K. Ghanty, and Arup Banerjee J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b11797 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Remarkable Structural Effect on the Gold-Hydrogen Analogy in Hydrogen-Doped Gold Cluster Megha,†,‡ Chinnathambi Kamal,†,‡ Krishnakanta Mondal,¶ Tapan K. Ghanty,∗,‡,§ and Arup Banerjee∗,†,‡ †Human Resources Development Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India ‡Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 450094, India ¶Department of Physics, Indian Institute of Science Education and Research, Pune 411008, India §Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In accordance with the well established gold-hydrogen analogy a hydrogen atom mimics the properties of a gold atom in gold clusters. In a recent study it has been demonstrated that the properties of a hydrogen atom doped small gold cluster (Au7 H) are not in conformity with the aforementioned analogy. In this paper we study the properties of Au7 H cluster exhaustively to re-examine the validity of gold-hydrogen analogy in the context of adsorption of CO and O2 molecules on pristine gold and hydrogen-atom doped gold clusters. For this purpose we first determine the most stable structure of Au7 H cluster by using ab initio density functional theory based method with generalized gradient approximation (GGA) and Meta-GGA exchangecorrelation functionals. We carry out geometry optimization by considering various planar and three-dimensional isomers of Au7 H cluster as initial geometries. We find that the lowest energy structure of Au7 H is a planar one with C2v symmetry and it is very close to the structure of Au8 cluster with D4h symmetry. Furthermore, to examine the validity of gold-hydrogen analogy we carry out a detailed investigation of the adsorption of CO and O2 molecules on the most stable as well as various other low energy isomers of Au7 H cluster. We find that the adsorption energies and the extent of activation of CO and O2 molecules on the most stable planar isomer of Au7 H are almost the same as those on the parent Au8 cluster with D4h symmetry proving the validity of gold-hydrogen analogy. On the other hand, for the high energy three-dimensional isomers of Au7 H cluster obtained from pristine Au8 cluster with Td symmetry, we find a significant enhancement in adsorption energy as well as the extent of activation of CO and O2 molecules as compared to those for the corresponding pristine cluster. Therefore, the high reactivity of the 3D isomer of Au7 H cluster may be attributed to its existence in a state which is higher in energy than its most stable planar isomer.

2


Page 2 of 34

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Nanoclusters made of gold atoms occupy a prime place in the field of atomic and molecular clusters due to their rich and interesting electronic, chemical, optical, and catalytic properties. 1–8 It is this catalytic activity of supported tiny gold particles towards various chemical reactions including economically vital and environmentally important oxidation of CO, as demonstrated by Haruta et al. 8 led to the birth of the field of nanocatalysis. 7 Since then numerous theoretical and experimental studies to understand the catalytic properties of gold based nanoclusters have been reported in the literature. These studies have clearly shown that the size and charge state of the cluster play a crucial role on the chemical reactivity, as depending on the size and charge state, the geometric and electronic properties of these clusters vary significantly. Furthermore, it has also been shown that the electronic properties of gold clusters can be modulated in desirable manner by incorporating a guest atom from transition or coinage or alkali metal groups. This in turn provides a convenient way to control the chemical reactivity of small sized gold clusters towards adsorption of various gas molecules useful from practical view points. For example, both theoretical and experimental studies have revealed that the transition metal atoms like Ti, V, Pd, and Pt can significantly enhance the chemical reactivity of the host gold cluster towards adsorption of molecules like CO, O2 , and H2 O2 . 9–15 The effect of Li atom doping on the adsorption of CO molecule by small gold clusters (2-8 atoms) 16 and 20-atom gold cluster Au20 17 have also been reported in the literature. Besides the above mentioned atoms, the effect of doping gold clusters with a hydrogen atom on their electronic structure and chemical reactivity towards CO and O2 molecules have been studied theoretically by several researchers. 18–20 We note here that hydrogen atom possesses not only similar valence electronic configuration as that of gold atom but also both of them have similar electronegativity (2.20 for H and 2.54 for Au in Pauling scale). In Ref. 18, Jena et al. have demonstrated that H-atom doped small gold cluster Au7 H, exhibits enhanced reactivity towards adsorption of O2 molecule and also causes significant activation 3



of this molecule as compared to the corresponding pristine Au8 cluster. This is an interesting result as in contrast to this, the corresponding neutral pristine Au8 cluster binds very weakly with an O2 molecule. They have also shown that enhanced binding and activation of O2 molecule by Au7 H makes it a better catalyst for the environmentally important CO oxidation reactions as compared to Au8 cluster. These results are not only intriguing but also perplexing as they are not in conformity with the gold-hydrogen analogy where the chemistry of the gold atom resembles that of a hydrogen atom. 21–24 On the other hand, in a very recent study our group has demonstrated through ab initio DFT based calculations that the structure and electronic properties of a bigger H-doped gold cluster Au19 H remain very similar to those of pristine Au20 in line with the gold-hydrogen analogy. 20 Moreover, it is also observed that the adsorption energies and the extent of activation of CO and O2 molecules on Au19 H are very similar to the corresponding values for Au20 cluster. This strikingly different behaviour of Au7 H and Au19 H clusters towards CO and O2 molecules has motivated us to ask following question: why does Au7 H behave so differently as compared to another closed-shell but slightly bigger Au19 H cluster? Moreover, the clusters Au8 , Au7 H, Au20 , and Au19 H are magic clusters corresponding to the electronic shell closing with 8 and 20 electrons, and thus expected to behave in a similar way as far as chemical reactivity is concerned. To address this question and to provide in-depth insight into this issue we critically re-examine geometry, electronic, and chemical properties of Au7 H clusters by carrying out ab initio density functional theory (DFT) based calculations. For this purpose, we have considered various possible two-dimensional (2D) planar and three-dimensional (3D) geometric structures of Au7 H cluster. We also carry out systematic studies on the adsorption of CO and O2 molecules on various possible isomers of Au7 H cluster to resolve the above mentioned conundrum. Our study clearly reveals that the geometry of the cluster plays an important role in determining the adsorption properties of clusters. The rest of the paper is structured as follows: Section 2 presents a brief description of computational methodology employed in our work. Results of ab initio calculations per-

4


Page 4 of 34

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formed are discussed in Section 3. Lastly, Section 4 summarizes the present work.

Computational Details In the present work, all the calculations have been performed using DFT based method as implemented in Gaussian 16 program package. 25 While MWB60 basis set, 26,27 a 60-electron core pseudo potential (ECP) which also includes relativistic effects, is used for the gold atom, all other lighter atoms (i.e., H, C, and O) are treated with all-electron Def2TZVP basis set 28,29 as implemented in Gaussian 16 package. It is well known that the exchange-correlation (XC) functionals within the generalized gradient approximation (GGA) and Meta-GGA, which occupies a higher rung in Jacob ladder than GGA, 30,31 yield quite accurate results for the geometry of gold clusters. 17,32–35 Keeping this in mind, we employ Perdew-Burke-Ernzerhof (PBE) 36 and Tao-Perdew-Staroverov-Scuseria (TPSS) 37 XC functionals falling under GGA and Meta-GGA, respectively for all the calculations presented in paper. We use two XC functionals at different levels of approximation to check the consistency of the results presented in this paper as it is well known that adsorption characteristics are not accurately predicted by the GGA functionals. For each cluster, geometry is optimized using Berny algorithm with tight convergence criterion which set the threshold to 1.5×10−5 Hartee/Bohr for the forces, A for the displacement, and 1.7×10−9 Hartree for the energy change. To find the 6.0×10−5 ˚ ground state geometries, we consider various possible initial planar two-dimensional (2D) and non-planar three-dimensional (3D) structures of pristine and H-doped gold clusters. In order to check the stability of optimized structures calculations of vibrational frequencies are performed at the same level of theory. The stability of the structure is confirmed by the absence of imaginary value of vibrational frequency. We use Vesta software 38 for generating all the structures presented in this paper.

In order to establish the accuracy of the above mentioned basis set and the XC-functionals

5



Page 6 of 34

for H-doped gold clusters, we first carry out calculations for smaller gold-hydrogen systems, namely, dimers Au2 and AuH along with CO and O2 molecules. The results for the binding energy (BE), bond length and stretching frequency of the two dimers as well as CO and O2 molecules are presented in Table 1 along with the already available corresponding experimental and theoretical data. It can be seen from this Table 1 that our calculated values of BE, bond length, and stretching frequency, using PBE and TPSS XC-functionals and the basis sets (MWB60 and Def2TZVP), agree quite well with the theoretical and experimental data available in the literature. Thus, the basis set and the XC-functional chosen by us to carry out the calculations are quite accurate and we expect that they will yield sufficiently accurate results when employed for bigger Au7 H cluster. Table 1: Calculated and Experimental Values of the Bond Length, Binding Energy, and Streching Frequency of Au2 , AuH Dimers, and CO, O2 Molecules system Au2 AuH CO O2

bond length (˚ A) method present work reported PBE TPSS PBE TPSS PBE TPSS PBE TPSS

2.563 2.551 1.542 1.544 1.137 1.134 1.219 1.219

2.53a [2.472c ] 1.542e [1.5238g ] 1.136b [1.13i ] 1.24d [1.21c ]

binding energy (eV) present work reported 2.156 2.135 3.115 3.175 11.665 11.021 6.246 5.511

2.214a 2.298b [2.08d ] 3.15f [3.22g ] 11.65h [11.24j ] 6.55d [5.23c ]

frequency (cm−1 ) present work reported 168 173 2240 2249 2130 2140 1556 1552

171a [191c ] 2226.6e [2305g ] 2122b [2143i ] 1580.19k [1576.2l ]

a Ref., 39 b Ref., 14 c Ref., 40 d Ref., 41 e Ref., 42 f Ref., 20 g Ref., 43 h Ref., 44 i Ref., 45 j Ref., 46 k Ref., 47 l Ref. 48

The numbers within the square brackets indicate the experimental values.

Results and Discussion Geometries and Electronic properties of Au7 H cluster To study the electronic and chemical properties of Au7 H, first we need to determine the correct ground state structure of this cluster. To this end, we carry out DFT based geometry optimization calculations by considering numerous initial geometries of Au7 H cluster generated by replacing one of the symmetrically non-equivalent gold atom in each isomer of Au8 6


Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cluster considered in this paper by a hydrogen atom. At this point it is necessary to mention that the dimensionality of Au8 cluster that is, whether it assumes a two-dimensional (2D) planar structure or a three-dimensional (3D) structure in the ground state remained a long standing matter of debate. To resolve this issue several studies devoted to the calculations of optimized ground state geometry of Au8 cluster by employing quantum chemical 49–51 and DFT based methods 51–55 have been reported in the literature. The earlier study of Olson et al. 49 found 3D structure of Au8 to be the global minimum by employing MP2 and CCSD(T) level of theory. However, in a later work 50 with improved correlation consistent basis set for gold, Olson and Gordon found that MP2 level of theory still favours nonplanar 3D Au8 structure, whereas CCSD(T) level of theory increasingly favours planar structure with the improvement in the basis set. These results were also confirmed by a very recent study. 51 On the other hand, except for Ref. 49 all the DFT based ab initio calculations unanimously predicted a 2D planar structure as the most stable morphology of Au8 cluster. It is important to note here that recently it has also been verified experimentally through far-infrared vibrational spectroscopy in conjunction with DFT based calculations that the ground state structure of Au8 is a planar one with D4h symmetry. 56 Thus, to generate the initial geometries of Au7 H cluster and to make the search for the lowest energy structure more exhaustive we choose all the three lowest energy isomers of Au8 cluster predicted in Refs. 49,50,53,57 and replace non-equivalent gold atoms in each of them by a hydrogen atom. These three isomers are, one with planar structure of D4h symmetry and the other two with 3D structures having Td and D2d symmetries. For the sake of completeness first we optimize the geometry of Au8 cluster using DFT based ab initio method with both PBE and TPSS XC functionals. The optimized structures of the above mentioned isomers in ascending order of energy are provided in the Supporting Information. We find that energetically 2D structure with D4h symmetry is more stable than both the 3D structures. For example, PBE XC functional predicts 2D planar structure to be 0.306 eV and 0.440 eV more stable than 3D structures with Td and D2d symmetries, respectively. Similarly with TPSS XC functional it is found

7



that 2D planar structure is more stable than 3D structures with Td and D2d symmetries by around 0.089 eV and 0.221 eV, respectively. We note here that our results for various characteristics of the lowest energy isomer of Au8 cluster like binding energy, Au-Au bond length, and vibrational frequencies agree well with the corresponding experimental 56 and already published quantum chemical and DFT based data. 51,53–55 Next to make our search for the ground state structure of Au7 H more exhaustive, we also consider three lowest energy isomers of Au7 cluster 39,51,58,59 and generate initial geometries of Au7 H cluster by adding a single H atom at all possible symmetrically non-equivalent locations in each of them. For this purpose, we first optimize the structures of above mentioned three isomers of Au7 cluster at both PBE and TPSS levels, which are provided in the Supporting Information in ascending order of their energies. In conformity with earlier reported data 39,51,58,59 we also find that planar geometry with Cs symmetry represents the ground state of Au7 cluster. Having examined the various possible low energy structures of Au8 and Au7 clusters, we next proceed to find the optimized structure of Au7 H cluster. We generate the initial structures of Au7 H from various isomers of Au8 and Au7 clusters using the procedure described above. In this way we generate around 30 different geometrical structures of Au7 H cluster which are then used as initial geometries to carry out the geometry optimization calculations for Au7 H cluster. Subsequently, we also carry out vibrational frequency calculations for all the optimized structures of Au7 H clusters and the absence of imaginary frequency confirms their existence in stable state. For convenience, we denote various isomers of Au7 H cluster in accordance with their dimensionality namely, 2D n and 3D n (where n is an integer indicating order of energy, the higher value of n corresponds to a less stable isomer) for two- and three-dimensional geometries, respectively. In Figure 1, we display the doped Au7 H clusters optimized at the meta-GGA level with TPSS XC functional. In this figure we present only those structures which are having a maximum of about 1.5 eV energy difference with respect to the most stable planar 2D 1 isomer of Au7 H. In Table 2 we present the results for relative energy, average binding energy per atom (Eb ), average Au-Au bond length (dAu-Au ), Au-H

8


Page 8 of 34

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bond length (dAu-H ), and the energy difference (∆HL ) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for all the low energy isomers of doped Au7 H cluster calculated using TPSS XC functional. The corresponding results with PBE functional, which exhibit similar trend are provided in the Supporting Information. The average binding energy per atom, Eb (Au7 H), is calculated by using the expression Eb (Au7 H) = − [E(Au7 H) − 7E(Au) − E(H)] /8

(1)

where E(Au7 H), E(Au), and E(H) are energies of the Au7 H cluster, Au and H atoms, respectively. It is important to note here that a higher value of the average binding energy corresponds to the higher thermodynamic stability of the cluster. To further characterize the strength of binding of H atom with the cluster, we also calculate the amount of energy required to remove the H atom from the cluster by using following expression:

Eb (H) = − [E(Au7 H) − 7E(Au) − E(H) − 7Eb (Au8 )]

(2)

where E(Au7 H), E(Au), and E(H) are energies of the Au7 H cluster, Au and H atoms, respectively. In the above expression for Eb (H), the term Eb (Au8 ) denotes the average binding energy per atom of an Au8 cluster given by

Eb (Au8 ) = − [E(Au8 ) − 8E(Au)] /8

(3)

with E(Au8 ) and E(Au) denoting the energies of Au8 cluster and Au atom, respectively. In Equation (3) for E(Au8 ) we use the energy of the most stable isomer of Au8 cluster with D4h symmetry. We note here that a higher positive value of Eb (H) signifies a stronger binding of H atom with the cluster. The results for Eb (H) are also compiled in Table 2.

9



Page 10 of 34

C2v

C1

C5v

C2v

2D 1 (0.000, 0.000)

3D 1 (0.556, 0.480)

3D 2 (0.817, 0.627)

2D 2 (0.818, 0.762)

Cs

C2v

C3v

C3v

3D 3 (0.950, 0.842)

2D 3 (1.075, 1.247)

3D 4 (1.242, 1.103)

3D 5 (1.446, 1.413)

Figure 1: Structures of Au7 H cluster optimized with PBE XC functional. The symmetry of the isomers is given at the top of each structure. The values within the parenthesis indicate the relative energy (in eV) with respect to the most stable isomer of Au7 H with C2v symmetry obtained with PBE and TPSS XC functionals (first and second numbers respectively). Golden-yellow and blue balls represent Au and H atoms, respectively.

First of all we note that the results obtained with both PBE and TPSS XC functionals conclusively show that the planar isomer 2D 1 corresponds to the most stable structure of Au7 H cluster. From Figure 1, we observe that for most of the isomers (including the lowest energy isomer 2D 1), the dopant H atom binds with two neighbouring Au atoms in bridge configuration, which is consistent with the observation made in Refs. 19 and 60. Moreover, from Table 2 we find that for all the isomers Au-H bond length is higher than that of a AuH dimer (1.542 ˚ A). It can also be seen that for all the isomers, where H atom binds with two Au atoms, the bond length dAu-H is significantly higher than those in which H atom binds with a single Au atom (3D 1 and 3D 2). For the lowest energy structure 2D 1, the value of

10


Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au-H bond length is 1.750 ˚ A which is around 13.5 % higher than that of Au-H bond length in AuH dimer. Therefore, we conclude that H atom interacts strongly with a single Au atom than with the two or more atoms in the cluster. We also note that the average Au-Au bond length in the lowest energy isomer (2D 1) is slightly higher than that in the pristine Au8 cluster. Table 2: Relative Energy (Erel ), Au-Au Average Distance (dAu-Au ), Au-H Distance (dAu-H ) , Average Binding Energy per Atom (Eb ), Binding Energy of HAtom (Eb (H)), and HOMO-LUMO Gap (∆HL ) for 2D and 3D isomers of Au7 H cluster with TPSS XC-functional isomers 2D 3D 3D 2D 3D 2D 3D 3D

1 1 2 2 3 3 4 5

symmetry Erel (eV) C2v C1 C5v C2v Cs C2v C3v C3v

0.000 0.480 0.627 0.762 0.842 1.247 1.103 1.413

dAu-Au (˚ A) dAu-H (˚ A) 2.701 2.782 2.825 2.751 2.836 2.670 2.817 2.857

1.754 1.590 1.590 1.808 1.710 1.715 1.937 1.841

Eb (eV) Au7 H

Au8

1.969 1.909 1.891 1.874 1.864 1.813 1.831 1.792

1.888 1.860

1.888 1.877 1.877

Eb (H) (eV) 2.536 2.056 1.912 1.776 1.696 1.288 1.432 1.120

∆HL (eV) Au7 H

Au8

2.046 1.749 1.907 0.664 1.643 1.647 1.844 1.375

1.565 1.659

1.565 2.121 2.121

To characterize the stability of doped Au7 H cluster we focus our attention on the results for the relative energy, average binding energy per atom (Eb ), and binding energy of H atom (Eb (H)) as presented in Table 2. We find that like pristine Au8 cluster, the planar 2D structure (2D 1) of Au7 H is significantly more stable than the lowest energy 3D isomer (3D 1). For example, with TPSS XC functional, it is found that the lowest energy planar 2D structure of Au7 H is more stable than 3D 1 and 3D 2 isomers by around 0.480 eV and 0.627 eV, respectively. Similarly with PBE XC functional, the two lowest energy 3D isomers of Au7 H, namely 3D 1 and 3D 2 are respectively, around 0.556 eV and 0.817 eV higher in energy than the lowest energy planar isomer (2D 1) of Au7 H. Since the difference in energy of the lowest energy 3D isomer (3D 1) and the most stable planar 2D 1 isomer is quite high, it is expected that in experimental generation of these clusters planar isomer 2D 1 will be energetically well discriminated from other isomers of Au7 H cluster. These results clearly 11



demonstrate that the doping of H atom stabilizes the planar structure of Au7 H significantly over its 3D counterparts. Further, we observe from Table 2 that the average binding energy per atom is highest for the most stable isomer (2D 1) of Au7 H and it decreases as the energy of the isomer increases. Further, we note that the average binding energy per atom of the lowest energy isomer (2D 1) of Au7 H cluster is just marginally higher by around 0.08 eV (TPSS) as compared to the corresponding planar isomer of bare Au8 cluster. Therefore, with respect to the results for average binding energy per atom and the cluster structure we conclude that the planar form of Au7 H cluster clearly supports the gold-hydrogen analogy. Further, it can be seen from Table 2 that binding energy of H atom is highest (2.536 eV and 2.436 eV with TPSS and PBE, respectively) for the most stable planar isomer (2D 1) of Au7 H cluster. Moreover, the binding energy of H atom in Au7 H cluster exhibits a trend which is similar to that of average binding energy per atom. We note that the binding energy of H atom for the lowest energy 3D isomer (3D 1) of Au7 H cluster is 0.480 eV and 0.552 eV lower than that of the most stable 2D 1 isomer, with TPSS and PBE XC functionals, respectively. Therefore, from the above results we infer that the binding of the H atom is strongest in the most stable planar isomer (2D 1) of Au7 H cluster. Having discussed the stability of the Au7 H cluster, we next proceed with the discussion on the results for HOMO-LUMO gap which characterizes the chemical stability of the species. It can be seen from Table 2 that the most stable Au7 H cluster 2D 1 possesses the highest value of HOMO-LUMO gap of 2.046 eV (with TPSS functional) which is around 0.30 eV and 0.14 eV higher than the values of HOMO-LUMO gap of the next two most stable 3D isomers 3D 1 and 3D 2, respectively. Similar trends are obtained with PBE functional. It indicates that the planar isomer 2D 1 of Au7 H cluster is both thermodynamically (Eb ) and chemically (HOMO-LUMO gap) more stable than its 3D counterparts. In addition, we note that the value of the HOMO-LUMO gap of 2D 1 isomer of Au7 H cluster is significantly higher (by around 0.48 eV both with TPSS and PBE functional) than that of its parent planar Au8 cluster with D4h symmetry. From this result we infer that the substitution of a gold atom by

12


Page 12 of 34

Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a hydrogen atom leads to a considerable enhancement of HOMO-LUMO gap of Au7 H cluster over that of a pristine Au8 cluster, which is contrary to the gold-hydrogen analogy mentioned above. To gain more insight into the electronic properties of H-doped cluster we display in Figure 2, the frontier molecular orbitals of the lowest energy isomer of Au7 H cluster (2D 1) along with those of its parent Au8 cluster. It can be seen from this figure that HOMO of Au7 H looks very similar to that of Au8 except at the location of H atom which does not make any contribution to the HOMO. On the other hand, H atom makes significant contribution to the LUMO of Au7 H and the contribution of other Au atoms almost remains the same as compared to those in bare Au8 cluster. We note here that the peripheral Au atom opposite to the H atom contributes maximum to the LUMO of Au7 H cluster and consequently this site is expected to be the most reactive one.

13



Page 14 of 34

(b) Au7 H(2D 1)

(a) Au8 (D4h )

Figure 2: Molecular Orbital diagrams of HOMO and LUMO: (a) pristine Au8 (D4h ) and (b) H-atom-doped Au7 H(2D 1) clusters optimized with PBE XC-functional.

Before proceeding further, we wish to highlight that in Ref. 18, a 3D structure of Au8 cluster with Td symmetry was considered as starting geometry to obtain the ground state geometry of doped Au7 H cluster. The optimized ground state structure of Au7 H cluster reported in Ref. 18 and employed further for calculations of CO and O2 adsorption is similar to the 3D 4 isomer of Au7 H cluster shown in Figure 1. But from our calculation we find that the 3D 4 isomer represents a state of Au7 H cluster having 1.103 eV (with TPSS functional) and 1.242 eV (with PBE functional) higher energy with respect to the most stable 2D 1 planar isomer. Furthermore, unlike other isomers, the H atom in the 3D 4 structure is bonded with three neighbouring gold atom resulting into longest Au-H bond length of 14


Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.937 ˚ A. In contrast to the most stable 2D 1 isomer, the average binding energy per atom of 3D 4 isomer of Au7 H clusters is slightly lower than that of parent Au8 cluster. It is also interesting to note that unlike 2D 1 isomer, the value of HOMO-LUMO gap of the 3D 4 isomer of Au7 H cluster (1.844 eV with TPSS and 1.718 eV with PBE) is lower than that of corresponding parent Au8 cluster by around 0.277 eV and 0.306 eV with TPSS and PBE XC functionals, respectively. As mentioned above the lowering of HOMO-LUMO gap signifies that the non-planar isomer 3D 4 of Au7 H will be chemically less stable or more reactive as compared to its parent Au8 cluster with Td symmetry. This behaviour of 3D 4 isomer of Au7 H cluster is in contradiction with the trend obtained for the most stable planar isomer 2D 1. We conjecture that it is this decrease in HOMO-LUMO gap, which leads to the high reactivity of 3D 4 isomer of Au7 H cluster towards the adsorption of CO and O2 molecules as predicted in Ref. 18. This result has also motivated us to critically re-examine and compare the adsorption of CO and O2 molecules on the most stable planar isomer 2D 1 and energetically higher non-planar isomer 3D 4 of Au7 H cluster. In the next section we discuss the results of the calculations on adsorption of CO and O2 molecules on two different isomers (2D 1 and 3D 4) of Au7 H clusters. To check the accuracy and consistency of the results discussed above we also perform calculations with TPSSh 37,61 XC functional which falls in the category of hybrid Meta-GGA XC functional occupying a higher rung in the Jacob ladder as compared to TPSS and PBE XC functionals. We find that all the properties presented in Table 2 obtained with TPSSh XC functional exhibit similar trends as those calculated with TPSS and PBE XC functionals. The values of various properties calculated with TPSSh and TPSS XC functionals are also found to be very close to each other.

15



Page 16 of 34

Adsorption of CO and O2 molecules on Au7 H cluster To study the adsorption of CO and O2 molecules, first we consider the most stable planar isomer 2D 1 of Au7 H cluster and adsorb a CO molecule or an O2 molecule separately at four non-equivalent binding sites of the cluster. In order to characterize the interaction between CO/O2 molecule and Au7 H/Au8 cluster, we calculate adsorption energy Eads defined as

Eads = − [E(Cluster − CO/O2 ) − E(Cluster) − E(CO/O2 )]

(4)

where E(Cluster-CO/O2 ), E(Cluster), and E(CO/O2 ) are the energies of the cluster-CO/O2 complexes, parent cluster (Au7 H or Au8 ), and the bare CO/O2 molecule, respectively. We note here that a positive value of Eads , obtained using above expression (4), signifies binding with the cluster and its higher value implies a stronger interaction between the cluster and molecule.

16


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au8 (D4h )-CO

Au7 H(2D 1)-CO

Au8 (D4h )-O2

Au7 H(2D 1)-O2

Figure 3: Structures of 2D Au8 (D4h )-CO/O2 and Au7 H(2D 1)-CO/O2 Cluster-molecule complexes optimized with TPSS XC-functional. Golden-yellow, blue, brown, and red balls represent Au, H, C, and O atoms, respectively.

In the following we present the results obtained using TPSS XC functional and the results with PBE XC functional which show similar trends are provided in the Supporting Information. In Figure 3 we display the most stable structures of Au7 H-CO and Au7 H-O2 cluster-molecule complexes along with the structures of the corresponding Au8 -CO and Au8 O2 pristine cluster-molecule complexes. Our results for the adsorption of CO and O2 on planar Au8 cluster are consistent with the already published data. 41,62,63 Furthermore, the

17



Page 18 of 34

results for the adsorption energy of CO and O2 molecule on four non-equivalent binding sites of Au7 H cluster are presented in Table 3 and Table 4 respectively, along with the results for bond lengths (dC-O /dO-O ) and bond stretching frequencies (νC-O /νO-O ). For comparison, we also include the aforementioned results for CO and O2 adsorption on the parent planar Au8 cluster. Table 3: Adsorption Energy (Eads ), C-O Distance (˚ A), and C-O Streching Frequencies (νC-O ) for 2D geometries of Au8 (D4h )-CO and Au7 H(2D 1)-CO clustermolecule complexes calculated with TPSS XC-functional (for bare CO: dC-O = 1.134 ˚ A and νC-O = 2140 cm−1 ) Au8 (D4h )-CO

site

Au7 H(2D 1)-CO

Eads (eV) dC-O (˚ A) νC-O (cm−1 ) Au1 Au3 Au6 Au7

0.550 0.550 1.120 1.120

1.141 1.141 1.141 1.141

2076 2076 2091 2091

Eads (eV) dC-O (˚ A) 0.363 0.686 1.145 0.969

1.142 1.141 1.141 1.140

νC-O (cm−1 ) 2065 2086 2092 2094

To discuss the results for adsorption on different binding sites of the parent planar Au8 cluster with D4h symmetry, we categorize these sites into two distinct groups based on their symmetry, namely, the four atoms (see Figure 3) located on the inner square (consisting of gold atoms Au1, Au2, Au3, and Au4) and the four peripheral atoms (Au5, Au6, Au7, and Au8). With the introduction of H atom, the symmetry changes from D4h to C2v which leads to increase in the number of symmetrically non-equivalent binding sites from two to four: group 1 (Au1 and Au2), group 2 (Au3 and Au4), group 3 (Au5 and Au7), and group 4 (Au6). Now we focus our attention on the results presented in Table 3 for adsorption of a CO molecule on Au7 H and Au8 clusters. We note that CO molecule prefers to get adsorbed on gold atom rather than on the dopant H atom. This result is consistent with the observations made in Ref. 16. We find that CO molecule binds with a single Au atom of the cluster in CO-mode, which involves interaction of cluster with the molecule through C atom. We observe from Table 3 that for planar Au8 cluster, the peripheral Au atoms show

18


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stronger binding with CO molecule than the inner Au atoms by around 0.570 eV. This result is in conformity with the observation made above regarding reactivity of the peripheral Au atoms in connection with the structure of LUMO shown in Figure 2a. This may also be attributed to the steric hinderence caused by the peripheral Au atoms. Moreover, we note that similar results were reported earlier for bigger Au13 cluster. 64 On the other hand, for Au7 H, one of the peripheral Au atom which is located farthest from H atom (Au6) shows highest adsorption energy and its value is marginally higher (0.025 eV with TPSS) than the corresponding energy in pristine Au8 cluster. In contrast to this, the adsorption energy at other two peripheral atoms (Au5, Au7) undergoes reduction by around 0.15 eV. For inner Au atoms we find that the atoms, which are located closest to the H atom (Au3 and Au4), bind CO molecule with slightly higher adsorption energy as compared to same atoms in pristine Au8 cluster. A reverse trend is observed for the other two inner atoms (Au1 and Au2), which are located farther from H atom than the Au3 and Au4 atoms. These variations in adsorption energy at different sites of Au7 H cluster are consistent with the structure of its LUMO as shown in Figure 2b. From Table 3, we also observe that the values of C-O bond length (dC-O ) of the adsorbed CO molecule at various possible binding sites of Au7 H cluster remain very close to the corresponding bond lengths in a pure Au8 cluster. This is also reflected in the results for C-O vibrational frequency, νC-O . On comparing the adsorption behaviour of CO molecule on Au8 and Au7 H clusters, we find that the highest CO adsorption energy (at Au6 site) is almost the same for both the clusters (1.120 vs 1.145 eV). Similarly, the C-O stretching frequency and the C-O bond length (for CO adsorbed at Au6 site) are virtually the same in both the clusters. All these trends clearly indicate that the behaviour of Au7 H is reasonably similar to that of Au8 cluster. It not only strongly supports goldhydrogen analogy but also indicates that the reactivity of Au8 and Au7 H clusters are almost identical. From the results presented in Table 3 we infer that with respect to adsorption of CO molecule, the planar isomer of Au7 H cluster displays similar trends as its parent Au8 cluster satisfying gold-hydrogen analogy.

19



Page 20 of 34

˚), and O-O Streching FreTable 4: Adsorption Energy (Eads ), O-O Distance (A quencies (νO-O ) for 2D geometries of Au8 (D4h )-O2 and Au7 H(2D 1)-O2 clustermolecule complexes calculated with TPSS XC-functional (for bare O2 : dO-O = 1.219 ˚ A and νO-O = 1552 cm−1 ) Au8 (D4h )-O2

site

Eads (eV) dO-O (˚ A) Au1 Au3 Au6 Au7

0.025 0.025 0.180 0.180

1.219 1.219 1.245 1.245

Au7 H(2D 1)-O2

νO-O (cm−1 ) 1551 1551 1347 1347

Eads (eV) dO-O (˚ A) 0.024 0.021 0.172 0.122

1.219 1.219 1.241 1.238

νO-O (cm−1 ) 1551 1546 1370 1388

Having discussed the results for adsorption of CO molecule on Au7 H cluster next we focus our attention on the results for adsorption of O2 molecule which are compiled in Table 4. First we note that like CO molecule, an O2 molecule also binds with a single Au atom of Au8 /Au7 H cluster. From Table 4, we find that the binding of inner Au atoms (Au1, Au2, Au3, and Au4) of Au8 cluster with O2 molecule is very weak (Eads = 0.025 eV). On the other hand, the peripheral Au atoms (Au5, Au6, Au7, and Au8) of Au8 cluster bind with O2 molecule more strongly than the inner atoms. However, the magnitude of adsorption energy at various sites in Au8 -O2 complex is significantly lower than those of Au8 -CO complex. Our results for Au8 -O2 complex match reasonably well with the results available in Ref. 62. It can be seen from Table 4 that the values of adsorption energy of O2 molecule with the inner Au atoms of Au7 H cluster almost remain the same as that of Au8 case. We also observe from Table 4 that like CO molecule, the adsorption energy of O2 molecule at the peripheral Au atom of Au7 H cluster (Au6), which is located farthest from the H atom is the highest. However, unlike CO adsorption case, the value of the highest adsorption energy of O2 molecule on Au7 H (0.172 eV) cluster is slightly lower than that of on parent Au8 cluster (0.180 eV). Moreover, from Figure 3, we infer that the structure of Au8 O2 cluster is somewhat deformed which was also observed in previous studies. 41,62 We estimate the deformation energy for this structure by removing O2 from optimized structure of Au8 O2 cluster and performing a single-point

20


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


calculation. We find that the energy difference between the deformed Au8 structure and the most stable structure of Au8 (with D4h symmetry) is only 7.81 meV. This small value of the deformation energy indicates that distortion of the structure costs a small amount of energy and therefore does not play a significant role in the binding of O2 molecule with Au7 H cluster. Furthermore, at the location of the highest adsorption energy on Au7 H cluster, we find that the O-O bond length and corresponding stretching frequency reduce nominally by around 0.004 ˚ A and 23 cm−1 respectively, in comparison with the corresponding values for the adsorption on parent Au8 cluster. We conclude from the results discussed above that as far as the reactivity of CO and O2 molecule with the most stable planar isomer of Au7 H cluster is concerned, the chemical behaviour of H atom is found to be quite similar to an Au atom in the cluster, which implies that the planar isomer satisfies the gold-hydrogen analogy. Having established the applicability of gold-hydrogen analogy for the most stable planar isomer of Au7 H cluster, in the following we focus our attention on the three-dimensional isomer (3D 4) of Au7 H cluster, which is obtained from the tetrahedral isomer of Au8 cluster. We point out here that the reactivity of the same isomer towards O2 adsorption has already been studied and reported by Jena et al. 18 They showed that this isomer exhibits remarkable enhancement in binding energy with an O2 molecule and also leads to significant increase in O-O bond length in comparison to the adsorption on a pristine tetrahedral Au8 cluster, clearly infringing gold-hydrogen analogy. Before discussing the results, we note that (see Table 2) the 3D 4 isomer of Au7 H cluster is around 1.242 eV higher in energy as compared to its most stable planar 2D 1 isomer. Moreover, we find that the cluster-molecule complexes Au7 H-O2 and Au7 H-CO obtained from the 3D 4 isomer are around 0.953 eV and 0.835 eV higher in energy than the corresponding cluster-molecule complexes obtained from the planar 2D 1 isomer, respectively. In Figure 4 we show the lowest energy structures of clustermolecule complexes obtained from 3D 4 isomer of Au7 H and tetrahedral isomers of Au8 cluster. The results for the adsorption characteristics of these cluster-molecule complexes are compiled in Table 5. For comparison we also present the corresponding results for the

21



Page 22 of 34

parent Au8 -CO and Au8 -O2 complexes.

(a) Au7 H-O2

(b) Au8 -O2

(c) Au7 H-CO

(d) Au8 -CO

Figure 4: Structures of Au7 H(3D 4)-CO/O2 and Au8 (Td )-CO/O2 Cluster-molecule complexes optimized with TPSS XC-functional. Golden-yellow, blue, brown, and red balls represent Au, H, C, and O atoms, respectively.

First we focus our attention on the adsorption of O2 molecule on the clusters. We observe from Table 5 that unlike the adsorption characteristics on planar isomer, the adsorption energy of an O2 molecule on 3D 4 isomer of Au7 H cluster is significantly higher (around 6 times) than that of on a corresponding pristine Au8 cluster with Td symmetry. We also note that this significant enhancement of adsorption energy for 3D 4 isomer of Au7 H cluster is also accompanied with an increase in O-O bond length and decrease in the O-O vibrational frequency giving rise to a large degree of O-O bond activation. We find that with doping of H atom in gold cluster, the O-O bond length increases by around 0.012 ˚ A, O2 adsorption energy increases by 0.266 eV, and O-O vibrational frequency decreases by around 64 cm−1 in comparison to the case when an O2 molecule is adsorbed on pristine Au8 cluster with Td symmetry. In contrast to this, the adsorption characteristics of an O2 molecule on the lowest energy planar isomers of Au7 H and Au8 clusters are very similar. It is important to emphasize that similar results are obtained with PBE functional which are provided in the Supporting Information. The strikingly different reactivity of 3D 4 isomer of Au7 H cluster in comparison to its 2D 1 isomer towards the adsorption of O2 molecule may be attributed to the fact that the former has significantly lower value of vertical ionization potential (7.596 22


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


eV with TPSS and 7.696 eV with PBE) as compared to the latter (8.057 eV with TPSS and 8.139 eV with PBE). The lower value of vertical ionization potential of 3D 4 isomer of Au7 H cluster may lead to a more efficient transfer of electron from its HOMO to the LUMO of O2 molecule. The results for the adsorption of O2 molecule on 3D 4 isomer of Au7 H are in conformity with the results reported by Jena et al. 18 for the same isomer. ˚), and O-O and Table 5: Adsorption Energy (Eads ), O-O and C-O Distances (A C-O Streching Frequencies (νO-O ) for 3D (generated from Td ) and 2D (generated from D4h ) geometries of cluster-CO/O2 cluster-molecule complexes calculated with TPSS XC-functional Au8 -O2 Eads (eV) dO-O (˚ A) 3D 4 2D 1

0.055 0.180

1.233 1.245

Au7 H-O2 νO-O (cm−1 ) 1415 1347

Eads (eV) dO-O (˚ A) νO-O (cm−1 ) 0.321 0.172

Au8 -CO 3D 4 2D 1

1.245 1.241

1351 1370

Au7 H-CO

Eads (eV)

dC-O (˚ A)

νC-O (cm−1 )

Eads (eV)

dC-O (˚ A)

νC-O (cm−1 )

0.813 1.120

1.142 1.141

2075 2091

1.413 1.145

1.142 1.141

2088 2092

Next we discuss the results for adsorption of CO molecule on 3D 4 isomer of Au7 H cluster. We find that the binding energy of CO molecule with 3D 4 isomer of Au7 H cluster gets enhanced by around two times as compared to the adsorption on the corresponding pristine Au8 cluster. However, unlike O2 adsorption, the adsorption of CO molecule on 3D 4 isomer leads to almost no change in the C-O bond length and C-O vibrational frequency as compared to the case in which a CO molecule is adsorbed on a pristine Au8 cluster. Like O2 case, for CO adsorption we find that above trends are irrespective of the XC functional used for carrying out DFT based calculations. This highly reactive characteristic of 3D 4 isomer of Au7 H cluster towards the adsorption of O2 and CO molecules is responsible for its highly efficient catalytic performance of H-doped Au7 H cluster for CO oxidation reaction in variance to the gold-hydrogen analogy. 18

23



From these results we infer that the adsorption of O2 and CO molecules on 3D 4 isomer of Au7 H cluster does not strictly abide by the gold-hydrogen analogy. On the other hand, most of the properties of the lowest energy planar 2D 1 isomer of Au7 H cluster are in conformity with the gold-hydrogen analogy. Therefore, we conclude that geometric structure plays a crucial role on the validity of the gold-hydrogen analogy in this H-doped Au7 H cluster.

Conclusion In this paper we re-examine the validity of gold-hydrogen analogy in small sized hydrogen doped gold cluster, viz., Au7 H cluster. To this end we employ a density functional theory based method with XC functional at two different levels, namely GGA (PBE) and Meta-GGA (TPSS) for calculating geometric and electronic properties, and adsorption characteristics of CO and O2 molecules on this cluster. To determine the lowest energy structure of Au7 H cluster, we consider around 30 different initial structures having 2D planar and 3D geometries and performed geometry optimization followed by vibrational frequency analysis. We find that like pristine Au8 cluster, the most stable structure of a doped Au7 H is a planar one with C2v symmetry. Moreover, the lowest energy 3D isomer with C1 symmetry of Au7 H is significantly higher in energy (around 0.48 eV and 0.56 eV with TPSS and PBE, respectively) than the most stable planar isomer. We find that the geometric structure, binding energy, and the shape of HOMO/LUMO orbitals of the most stable planar isomer of Au7 H are very close to the corresponding properties of the parent Au8 cluster with D4h symmetry indicating a hydrogen-like chemical behaviour of gold. Besides the above mentioned properties, we also examine the validity of gold-hydrogen analogy by investigating the adsorption of CO and O2 molecules on the Au7 H cluster. We find that the C-O bond length and its stretching frequency are only marginally different from the corresponding values for Au8 -CO complex. Similarly, almost no change in the adsorption energy of O2 molecule on Au7 H cluster is

24


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed in comparison to the adsorption on the pristine Au8 cluster. These results clearly indicate that the gold-hydrogen analogy is valid for the most stable planar isomer of Au7 H cluster. We also examine the adsorption of CO and O2 molecules on the high energy isomer (3D 4) of Au7 H cluster, which has been shown to possess properties not falling in line with the gold-hydrogen analogy. 18 We find in conformity with the earlier reported results that CO and O2 molecules get adsorbed on 3D 4 isomer of Au7 H cluster with enhanced adsorption energy and a greater activation of the adsorbed molecules in comparison to the adsorption of the same molecules on pristine Au8 cluster with Td symmetry. This behaviour of 3D4 isomer of Au7 H cluster is clearly in contradiction with the gold-hydrogen analogy and it is distinctly different from that of the lowest energy isomer of A7 H clutser. We conclude from present study that the gold-hydrogen analogy is obeyed by the most stable planar isomer of hydrogen doped Au7 H gold cluster and higher energy structures may fail to satisfy the analogy. Moreover, from the present study on Au7 H cluster and our earlier work on Au19 H cluster, 20 we conjecture that in general, properties of the most stable ground state structure of hydrogen doped gold clusters will be in accordance with the gold-hydrogen analogy. We are currently carrying out a systematic study of various electronic and chemical properties of hydrogen-doped gold clusters of different sizes to verify the conjecture.

Supporting Information Available The structures of Au8 and Au7 clusters, and the binding energy, bond length, and HOMOLUMO gap for various isomers of Au7 H cluster along with the data for adsorption of CO and O2 molecules on Au7 H and Au8 clusters with PBE XC-functional are given. This material is available free of charge via the Internet at http://pubs.acs.org/.

25



Acknowledgement Megha gratefully acknowledges HBNI RRCAT for financial support. We thank Dr. P. A. Naik, Director RRCAT for his support. We appreciate the support of Computer Centre, RRCAT for providing computation facilities.

References (1) Pyykkö, P.; Runeberg, N. Icosahedral WAu12 : A Predicted Closed-Shell Species, Stabilized by Aurophilic Attraction and Relativity and in Accord with the 18-Electron Rule. Angew. Chem. Int. Ed. 2002, 41, 2174–2176. (2) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Häkkinen, H.; Barnett, R. N.; Landman, U. When Gold Is Not Noble:Nanoscale Gold Catalysts. J. Phys. Chem. A 1999, 103, 9573–9578. (3) Yoon, B.; Koskinen, P.; Huber, B.; Kostko, O.; von Issendorff, B.; Häkkinen, H.; Moseler, M.; Landman, U. Size-Dependent Structural Evolution and Chemical Reactivity of Gold Clusters. ChemPhysChem 2007, 8, 157–161. (4) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (5) Morton, S. M.; Silverstein, D. W.; Jensen, L. Theoretical Studies of Plasmonics using Electronic Structure Methods. Chem. Rev. 2011, 111, 3962–3994. (6) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153–166. (7) Heiz, U.; Landman, U. Nanocatalysis; Springer-Verlag Berlin Heidelberg, 2007. (8) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature Far Below 0o C. Chem. Lett. 1987, 16, 405–408. 26


Page 26 of 34

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Koyasu, K.; Mitsui, M.; Nakajima, A.; Kaya, K. Photoelectron Spectroscopy of Palladium-Doped Gold Cluster Anions; Aun Pd (n=1-4). Chem. Phys. Lett. 2002, 358, 224–230. (10) Koyasu, K.; Naono, Y.; Akutsu, M.; Mitsui, M.; Nakajima, A. Photoelectron Spectroscopy of Binary Au Cluster Anions with a Doped Metal Atom: Aun M (n=2-7), M=Pd, Ni, Zn, Cu, and Mg. Chem. Phys. Lett. 2006, 422, 62–66. (11) Morrow, B. H.; Resasco, D. E.; Striolo, A.; Nardelli, M. B. CO Adsorption on Noble Metal Clusters: Local Environment Effects. J. Phys. Chem. C 2011, 115, 5637–5647. (12) Sadek, M. M.; Wang, L. Effect of Adsorption Site, Size, and Composition of Pt/Au Bimetallic Clusters on the CO Frequency: A Density Functional Theory Study. J. Phys. Chem. A 2006, 110, 14036–14042. (13) Song, C.; Ge, Q.; Wang, L. DFT Studies of Pt/Au Bimetallic Clusters and Their Interactions with the CO Molecule. J. Phys. Chem. B 2005, 109, 22341–22350. (14) Mondal, K.; Banerjee, A.; Ghanty, T. K. Structural and Chemical Properties of Subnanometer-Sized Bimetallic Au19 Pt Cluster. J. Phys. Chem. C 2014, 118, 11935– 11945. (15) Mondal, K.; Banerjee, A.; Fortunelli, A.; Ghanty, T. K. Does Enhanced Oxygen Activation always Facilitate CO Oxidation on Gold Clusters? J. Comput. Chem. 2015, 36, 2177–2187. (16) Jena, N. K.; Chandrakumar, K. R. S.; Ghosh, S. K. Theoretical Investigation on the Structure and Electronic Properties of Hydrogen- and Alkali-Metal-Doped Gold Clusters and Their Interaction with CO: Enhanced Reactivity of Hydrogen-Doped Gold Clusters. J. Phys. Chem. C 2009, 113, 17885–17892.

27



Page 28 of 34

(17) Mondal, K.; Manna, D.; Ghanty, T. K.; Banerjee, A. Significant Modulation of CO Adsorption on Bimetallic Au19 Li Cluster. Chem. Phys. 2014, 428, 75–81. (18) Jena, N. K.; Chandrakumar, K. R. S.; Ghosh, S. K. Beyond the Gold–Hydrogen Analogy: Doping Gold Cluster with H-atom–O2 Activation and Reduction of the Reaction Barrier for CO Oxidation. J. Phys. Chem. Lett. 2011, 2, 1476–1480. (19) Manzoor, D.; Pal, S. Hydrogen Atom Chemisorbed Gold Clusters as Highly Active Catalysts for Oxygen Activation and CO Oxidation. J. Phys. Chem. C 2014, 118, 30057–30062. (20) Mondal, K.; Agrawal, S.; Manna, D.; Banerjee, A.; Ghanty, T. K. Effect of Hydrogen Atom Doping on the Structure and Electronic Properties of 20-Atom Gold Cluster. J. Phys. Chem. C 2016, 120, 18588–18594. (21) Buckart, S.; Ganteför, G.; Kim, Y. D.; Jena, P. Anomalous Behavior of Atomic Hydrogen Interacting with Gold Clusters. J. Am. Chem. Soc. 2003, 125, 14205–14209. (22) Ghanty, T. K. Gold Behaves as Hydrogen: Prediction on the Existence of a New Class of Boron-Containing Radicals, AuBX (X=F, Cl, Br). J. Chem. Phys. 2005, 123, 241101– 1–241101–4. (23) Kiran, B.; Li, X.; Zhai, H.-J.; Wang, L.-S. Gold as Hydrogen: Structural and Electronic Properties and Chemical Bonding in Si3 Au3 +/0/− and Comparisons to Si3 H3 +/0/− . J. Chem. Phys. 2006, 125, 133204–1–133204–7. (24) Lauher, J. W.; Wald, K. Synthesis and Structure of TriphenylphosphinegoldDodecacarbonyltricobaltiron ([FeCo3 (CO)12 AuPPh3 ]):

A Trimetallic Trigonal-

Bipyramidal Cluster. Gold Derivatives as Structural Analogs of Hydrides. J. Am. Chem. Soc. 1981, 103, 7648–7650.

28


Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H. et al. Gaussian16 Revision B.01. 2016; Gaussian Inc. Wallingford CT. (26) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-Adjusted ab initio Pseudopotentials for the Second and Third Row Transition Elements. Theor. Chim. Acta 1990, 77, 123–141. (27) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P. D. W. Relativistic Effects in Gold Chemistry. I. Diatomic Gold Compounds. J. Chem. Phys. 1989, 91, 1762–1774. (28) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (29) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057–1065. (30) Perdew, J. P.; Schmidt, K. Jacob0 s Ladder of Density Functional Approximations for the Exchange-Correlation Energy. AIP Conf. Proc. 2001, 577, 1–20. (31) Perdew, J. P.; Ruzsinszky, A.; Tao, J.; Staroverov, V. N.; Scuseria, G. E.; Csonka, G. I. Prescription for the Design and Selection of Density Functional Approximations: More Constraint Satisfaction with Fewer Fits. J. Chem. Phys. 2005, 123, 062201–1–062201– 9. (32) Jiang, D.-e.; Whetten, R. L. Magnetic Doping of a Thiolated-Gold Superatom: FirstPrinciples Density Functional Theory Calculations. Phys. Rev. B 2009, 80, 115402–1– 115402–5.

29



(33) Zhang, X. D.; Guo, M. L. Electronic Structure and Optical Transitions of Au20−X CuX Nanoclusters. J. Nanosci. Nanotechnol. 2010, 10, 7192–7195. (34) Beletskaya, A. V.; Pichugina, D. A.; Shestakov, A. F.; Kuz0 menko, N. E. Formation of H2 O2 on Au20 and Au19 Pd Clusters: Understanding the Structure Effect on the Atomic Level. J. Phys. Chem. A 2013, 117, 6817–6826. (35) Ma, W.; Chen, F. CO Oxidation on the Ag-Doped Au Nanoparticles. Catal. Lett. 2013, 143, 84–92. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (37) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401–1–146401–4. (38) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272–1276. (39) Deka, A.; Deka, R. C. Structural and Electronic Properties of Stable Aun (n=2-13) Clusters: A Density Functional Study. J. Mol. Struct.: THEOCHEM 2008, 870, 83– 93. (40) Morse, M. D. Clusters of Transition-Metal Atoms. Chem. Rev. 1986, 86, 1049–1109. (41) Fernández, E. M.; Ordejón, P.; Balbás, L. C. Theoretical Study of O2 and CO Adsorption on Aun Clusters (n=5-10). Chem. Phys. Lett. 2005, 408, 252–257. (42) Wang, X.; Andrews, L. Gold Hydrides AuH and (H2 )AuH and the AuH3 Transition State Stabilized in (H2 )AuH3 : Infrared Spectra and DFT Calculations. J. Am. Chem. Soc. 2001, 123, 12899–12900.

30


Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (44) Wu, X.; Senapati, L.; Nayak, S. K.; Selloni, A.; Hajaligol, M. A Density Functional Study of Carbon Monoxide Adsorption on Small Cationic, Neutral, and Anionic Gold Clusters. J. Chem. Phys. 2002, 117, 4010–4015. (45) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, USA, 2001. (46) Chase, M. W.; Jr.,; Davies, C. A.; Downey, J. R.; Frurip, J. D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables Third Edition. J. Phys. Chem. Ref. Data 1985, 14, Suppl. 1. (47) Gadzhiev, O. B.; Ignatov, S. K.; Kulikov, M. Y.; Feigin, A. M.; Razuvaev, A. G.; Sennikov, P. G.; Schrems, O. Structure, Energy, and Vibrational Frequencies of Oxygen Allotropes On (n ≤ 6) in the Covalently Bound and van der Waals Forms: Ab Initio Study at the CCSD(T) Level. J. Chem. Theory Comput. 2013, 9, 247–262. (48) Broida, H. P.; Peyron, M. Emission Spectra of N2 , O2 , and NO Molecules Trapped in Solid Matrices. J. Chem. Phys. 1960, 32, 1068–1071. (49) Olson, R. M.; Varganov, S.; Gordon, M. S.; Metiu, H.; Chretien, S.; Piecuch, P.; Kowalski, K.; Kucharski, S. A.; Musial, M. Where Does the Planar-to-Nonplanar Turnover Occur in Small Gold Clusters? J. Am. Chem. Soc. 2005, 127, 1049–1052. (50) Olson, R. M.; Gordon, M. S. Isomers of Au8 . J. Chem. Phys. 2007, 126, 214310–1– 214310–6. (51) Baek, H.; Moon, J.; Kim, J. Benchmark Study of Density Functional Theory for Neutral Gold Clusters, Aun (n=2-8). J. Phys. Chem. A 2017, 121, 2410–2419.

31



(52) Han, Y.-K. Structure of Au8 : Planar or nonplanar? J. Chem. Phys. 2006, 124, 024316– 1–024316–3. (53) Diefenbach, M.; Kim, K. S. Spatial Structure of Au8 :Importance of Basis Set Completeness and Geometry Relaxation. J. Phys. Chem. B 2006, 110, 21639–21642. (54) Assadollahzadeh, B.; Schwerdtfeger, P. A Systematic Search for Minimum Structures of Small Gold Clusters Aun (n=2-20) and Their Electronic Properties. J. Chem. Phys. 2009, 131, 064306–1–064306–11. (55) Serapian, S. A.; Bearpark, M. J.; Bresme, F. The Shape of Au8 : Gold Leaf or Gold Nugget? Nanoscale 2013, 5, 6445–6457. (56) Gruene, P.; Butschke, B.; Lyon, J. T.; Rayner, D. M.; Fielicke, A. Far-IR Spectra of Small Neutral Gold Clusters in the Gas Phase. Z. Phys. Chem. 2014, 228, 337–350. (57) Hansen, J. A.; Piecuch, P.; Levine, B. G. Communication: Determining the LowestEnergy Isomer of Au8 : 2D, or not 2D. J. Chem. Phys. 2013, 139, 091101–1–091101–4. (58) Walker, A. V. Structure and Energetics of Small Gold Nanoclusters and Their Positive Ions. J. Chem. Phys. 2005, 122, 094310–1–094310–12. (59) Sekhar De, H.; Krishnamurty, S.; Pal, S. Understanding the Reactivity Properties of Aun (6 ≤ n ≤ 13) Clusters Using Density Functional Theory Based Reactivity Descriptors. J. Phys. Chem. C 2010, 114, 6690–6703. (60) Phala, N. S.; Klatt, G.; van Steen, E. A DFT Study of Hydrogen and Carbon Monoxide Chemisorption onto Small Gold Clusters. Chem. Phys. Lett. 2004, 395, 33–37. (61) Staroverov, V. N.; Scuseria, G. E.; Tao, J.; Perdew, J. P. Comparative Assessment of a New Nonempirical Density Functional: Molecules and Hydrogen-Bonded Complexes. J. Chem. Phys. 2003, 119, 12129–12137.

32


Page 32 of 34

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(62) Liao, M.-S.; Watts, J. D.; Huang, M.-J. Theoretical Comparative Study of Oxygen Adsorption on Neutral and Anionic Agn and Aun Clusters (n=2-25). J. Phys. Chem. C 2014, 118, 21911–21927. (63) Molina, L. M.; Hammer, B. Oxygen Adsorption at Anionic Free and Supported Au Clusters. J. Chem. Phys. 2005, 123, 161104–1–161104–5. (64) Amft, M.; Johansson, B.; Skorodumova, N. V. Influence of the Cluster Dimensionality on the Binding Behavior of CO and O2 on Au13 . J. Chem. Phys. 2012, 136, 024312– 1–024312–6.

33



Graphical TOC Entry

Au7 H (C3v )

Au7 H (C2v )

Which isomer will follow gold-hydrogen analogy?

34


Page 34 of 34

Remarkable Structural Effect on the GoldâHydrogen Analogy in

Recommend Documents

Remarkable Structural Effect on the GoldâHydrogen Analogy in